(Mostly) revert "Cleanup session on delete"
(Mostly) revert "Cleanup session on delete"

This reverts commit 1f26c5736949e1c3b57c024a315e33fc419f126e for py2.
Apparently the existence of the __del__ method on Python 2 prevents us
from cleaning up all file descriptors.

Change-Id: Id6cff5dd7b9faf9c4240c0cb26b74d05ed37da5b
Closes-Bug: #1873435
Related-Bug: #1838775

openstack/python-swiftclient,sohonetlabs/python-swiftclient,sohonetlabs/python-swiftclient,openstack/python-swiftclient